Two Turkish F-16 fighter jets were scrambled after a Syrian army helicopter approached the border town of Azmarin and fired down onto rebel lines.

The helicopter left shortly after the Turkish planes arrived, but according to one official, two Syrian jets were seen in the distance immediately afterwards.

Nato leaders are warning Turkey against escalating its diplomatic offensive against Syria, which has already seen it send artillery fire into Bashar al-Assad's territory in retaliation for mortar fire from the Syrian side.

The crisis entered a new phase on Wednesday when Turkey forced down a Syrian passenger plane over Ankara, claiming it was carrying Russian arms for the Assad regime.

Moscow admitted on Friday that the plane was carrying radar equipment to Syria, but insisted that did not justify Turkey's behaviour.

Turkey has backed the uprising against President Assad from the start, but has become increasingly concerned at the prospect of the war shifting over its own borders. Much of the most intense fighting is now in two provinces close to its border, Idlib and Aleppo.

Azmarin is so close to that border it can be viewed easily from the Turkish side. Yesterday, residents of the Turkish village of Hacipasa stood on the roofs of their houses and watched as thick plumes of smoke rose from the town.

Intense machine gunfire could be heard, while through their camera lenses photographers could pick out rebel fighters on streets corners and dashing for cover. A sniper was visible in one building.

Rebels claim to be in control of Azmarin, but to be facing a counter-attack from regime forces, including from the air. The helicopter seen from the Turkish side is thought to have fired onto the town 10-15 times before the Turkish jets arrived.

It was not the first time Turkish jets had been deployed. They were sent to the same stretch of border in the summer as fighting approached it, after another Turkish jet that had briefly crossed into Syrian air space was shot down over the Mediterranean.

One Turkish official told The Daily Telegraph: "Under the rules of engagement chosen by us, if we feel there is something coming to the border, sometimes our guys scramble." The official said the fighting along Turkey's 550-mile border with Syria "presented a dangerous situation".

The German foreign minister, Guido Westerwelle, is among world leaders heading to Ankara to discuss the crisis. He will meet his Turkish counterpart, Ahmet Davutoglu, on Saturday, while the United Nations and Arab League envoy, Lakhdar Brahimi, will also visit.

There was no let-up in the ongoing confrontation between Turkey and Russia over Ankara's grounding of the Syrian passenger plane on a scheduled flight from Moscow, even though the Russians now admit it contained radar parts which were of potential "dual military-civilian use". Sergei Lavrov, the Russian foreign minister, rejected the characterisation of them as "weapons".

"We have no secrets," he said. "There were, of course, no weapons on the plane and could not have been any. There was a cargo on the plane that a legal Russian supplier was sending in a legal way to a legal customer."

Russia refused to allow United Nations security council resolutions that would have imposed a binding arms embargo on Syria, and is not covered by the European Union embargo. Turkey said it acted under the Convention on International Civil Aviation, which says that civilian flights carrying "munitions of war or implements of war" have to have permission from countries on its route.

Russia's Kommersant newspaper said the plane was carrying 12 crates of Russian radar equipment for an air defence system.

Unnamed sources told the paper the items required no special documentation because they did not pose a danger to passengers. "These are not weapons," said one. "If a person takes a switched-off radio set on a plane, does that pose a threat to the craft or the people on board?"

The head of Russia's arms export company admitted in June that it continued to provide Syria with Buk-M2 anti-aircraft missiles and Pantsyr-S1 radar-guided surface-to-air missiles.

The Turkish prime minister, Recep Tayyip Erdogan, has yet to expand on his own claim that the shipment also contained ammunition.